Most color printing techniques use a so-called printing master such as a printing plate which carries an image and is mounted on the plate cylinder of a printing press. The printing master has a printing surface of which some areas are capable of accepting ink (the printing areas). In lithographic printing, the printing surface is a so-called lithographic surface consisting of oleophilic, ink accepting areas and oleophobic, ink repellent (non-printing) areas.
These printing techniques are binary processes wherein ink is transferred from the printing surface of the printing master to a (paper) substrate. Continuous tones are simulated by a so-called halftone image generated by a screening process. In conventional screening techniques the halftone image consists of dots which are equally spaced and vary in size. Frequency-modulated (also called stochastic) screening techniques use a different approach wherein small, equally-sized halftone dots are randomly placed and the observed density is determined by the number of these dots per square unit.
Multi-color prints are obtained by consecutively printing a limited set of process colors onto the substrate and each process color requires a separate printing master carrying the halftone image that is obtained by color separation and screening. The number of process colors may vary from two (e.g. black and one additional custom color such as the so-called spot-color printing process), to four (the widely used cyan, magenta, yellow and black subtractive primary colors) or even more (e.g. the so-called Hifi-color process). In order to obtain high quality prints these halftone color images need to be perfectly aligned. Misalignment, also called misregistration, may lead to printing artifacts such as moire (repetitive patterns often visible on a macroscopic scale). Though line or text printing requires no screening, bad registration of the printing masters can cause noticeable white gaps where colors should meet.
Nowadays printing masters are generally obtained by the so-called computer-to-film method (CtF) wherein various pre-press steps such as typeface selection, scanning, color separation and layout are accomplished digitally and the electronic files are transferred onto graphic arts film using an image-setter (one film for each process color). The processed film then can be used as a mask for the exposure of a plate precursor and after optional plate processing, a printing plate is obtained. The computer-to-plate method (CtP), also called direct-to-plate method, bypasses the creation of film and the digital document is transferred directly onto a plate precursor. In a special type of a computer-to-plate process, sometimes called `computer-to-plate-on-press` (CtPoP), the plate precursor is exposed after being mounted on the plate cylinder. CtPoP reduces the chance of misregistration significantly because no intermediate films are used and the plates are firmly mounted by clamps on the plate cylinder at the moment of exposure.
Even in the CtPoP method, several on-press phenomena may cause the plates to print out of register. Each plate cylinder is characterized by its specific mechanical properties and tolerances (pressure of contacting rollers, register pins clearance, driving mechanism etc.). In addition, the printing plate itself is not dimensionally stable during processing and printing, especially if the plate comprises a flexible, e.g. polyester or paper support. The plate is slightly stretched by tightening the clamping bolts of the plate cylinder and may relax afterwards. Still other parameters such as temperature changes and moistening by the ink and fountain solution (in lithographic printing) may cause dimensional changes of each printing plate. Therefore, quite some research has been carried out in order to find solutions that minimize the dimensional changes of printing plates.
EP-A-644,064 discloses a lithographic printing plate having on one side of a flexible support a layer comprising micro-particles of pressure sensitive adhesive covered with a protective stripping layer that is removed before mounting the plate on the plate cylinder. EP-A-690,349 discloses a method for making lithographic substrates having as support a laminate of aluminum and plastic. EP-A-807,534 also discloses a method to reduce the dimensional instability of the printing plate having a flexible support by laminating its back side (i.e. the side opposite to the printing surface carrying the image) to a base which is more dimensionally stable, e.g. a metallic base or a base made of a composite material comprising fibers and a resin matrix. Though these methods indeed may reduce the dimensional shifts of each individual plate, it gives no solution for adjusting the registration of the plates by controlling the dimensional correlation between multiple plates in a multi-color printing job.
In U.S. Pat. No. 5,531,162 a method is described for bringing plates in register that relies on a measuring device which uses the edges of the plate at the clamps of the plate cylinder as a reference. This method only provides a solution for controlling the plate deformation in one dimension perpendicular to the axis of the plate cylinder. The stretching of the plate upon tightening the clamping bolts causes a constriction of the plate along a line parallel to the axis of the plate cylinder, said constriction being maximal in the middle between both clamped plate edges. As a conclusion, there is a need for an improved method for printing plate registration.